Intermittent type drum advancing means in a high speed printer

ABSTRACT

A line printer comprising a set of type wheels that intermittently rotate in unison on a common shaft.

O United States Patent 1 [111 3,731,622 Baranoff 1 1 May 8, 1973 [541 INTERMITTENT TYPE DRUM 3,179,044 4/1965 Schierbeek ..101/93 c ADVANCING M AN N A I H 3,361,057 1/1968 Pm ....101/9 3 c 2,627,806 2/1953 Bealtie et a1. 2101/93 C SPEED PRINTER 3,139,818 7/1967 Koehn .101/93 C [76] Inventor: Albert J. Baranofl, c/o California 3,307,676 3/1967 Hickerson 101/93 C X Electro Scientific, 2203 S. Grand 3,528,368 9/ 1970 Oberholzer ..101/93 C Ave., Santa Ana, Calif. 92705 ,6 2 1/1958 ....101/93 C 3,426,676 2/1969 ....l0l/93 R Flledr 1970 3,390,988 3 1967 Touchman ....101 93 R 3,089,413 5/1963 MacNeill et al. ....l01/93 C [21] Appl' 20647 2,687,692 8/1954 Kubovy et a1; ..,1o1 110 [52] US. Cl. ..l0l/93 C, 101/110 Primary Examiner--Robert E. Pulfrey [51] Int. Cl ..B4lj 7/48, B41j 1/48 Assistant Examiner-E. M. Coven [58] Field of Search ..lO1/93 C, 93 R, 95, AttorneyCesari & McKenna [57] ABSTRACT [56] References Cited A line printer comprising a set of type wheels that in- UNITED STATES PATENTS termittently rotate in unison on a common shaft.

3,505,950 4/1970 Harper ..l01/93 C 18 Claims, 11 Drawing Figures WWW PATENTED 81913 3.731.622

SHEET 1 0F 7 INVENTOR (\l 5 g ALBERT J. BARANOFF CZMMMVh/QWQ ATTORNEYS PATEIIIEIT I 819B 3.731522 SHEET 3 OF 7 DRUM POSITION 24| OUTPUTS I POsITI N SENSOR I COLUMN I I I COMPARATOR I ENCODER I I f I I 242 I I COLUMN 2 I .I COMPARATOR I I I A l J L- I I I I 24I2 V V I I Q 1327 COLUMN I2 I INPUT T COMPARATOR I OEvICE I I Q I I HOME I 1 /11] STOP I24 OECIMAL ENABLE P ADVANCE I681 32! 34 ga w g? I SOLENIOD 1: I686 A HAMMER r SOLENIOD HAMMER SOLENOID sOLENOIO CLOCK FIgT 'IURN PO g ER INvENTOR FIG. 3 ALBERT LI. BARANOFF m am/wir/flzww ATTORNEYS PATENTEU 81913 3.731.622

SHEET 5 HF? INVENTOR ALBERT J. BARANOFF @cw'mM m ATTORNEYS PATENTEDHAY 8% 3,731,622

' SHEET 7 BF 7 g g Q 9 CD 9' LL INVENTOR ALBERT J. BARANOFF BY ATTORNEYS INTERMI'I'I'ENT TYPE DRUM ADVANCING MEANS IN A HIGH SPEED PRINTER FIELD OF THE INVENTION This invention relates to a line printer that prints successive lines of characters in response to character signals generated by an input device.

The outputs of most measuring devices or instruments are directly registered on analog indicators such as the dial type units commonly used to indicate voltages, weights and other often-measured quantities. More recently, direct digital indicators have been provided by means of electronic indicators such as the socalled nixie tubes.

If one desires to permanently display the output of an instrument, or indicate simultanously a number of such outputs, he can resort to several devices that display these outputs on paper or other suitable media. For analog indications the strip chart recorder provides these functions by tracing a continuous curve whose height above a base line at any point corresponds to the value of the measured parameter at a given time. For digital indications, various printers are used and it is a printer of this type to which the present invention is directed.

THE PRIOR ART One type of printer that can be used for this purpose is the teleprinter, a device that resembles a typewriter and, like a typewriter, prints a succession of characters extending along a line. However, in the context of an instrument display, the teleprinter is unduly large, mechanically complex and also rather expensive. Moreover, it is too slow for many applications.

There are also line printers which, in essence, print a whole line of characters at one time. One type of line printer has an array of type carriers in the form of bars or wheels positioned side-by-side, each bar or wheel carrying an entire set of characters. The type carriers are moved so that each one brings a selected character into position opposite the paper. The array is then forced against the paper so as to print the entire line. Again this printer is a rather complicated device and, furthermore, is too slow for many instrument applications.

Another line printer has a type drum or stack of type wheels that rotate continuously at high speed, each type wheel or drum segment corresponding to a character position along a line of characters. In each character position a hammer strikes the paper against the drum as the selected character for that position moves past the paper. Thus the varous characters along the line are not actually printed simultaneously, rather they are all printed during one complete rotation of the drum. This is perhaps the fastest mechanical printer in use at the present time. As such it is used to print the outputs of large electronic data processing machines that generate enormous amounts of output data. While conceptually simple, a printer of this type is a large expensive piece of equipment, far too bulky and expensive for many applications, including the printing of instrument outputs. Nor is its high speed needed for these applications.

OBJECT OF THE INVENTION A principal object of the invention is a printer suitable for recording the outputs of instruments and other devices generating outputs at comparable rates.

Another object of the invention is a printer having sufficient speed for the recorded parameter values to follow" variations in the parameters registered in the instrument outputs.

Yet another object is a printer that is simple in design, reliable, of small size and relatively inexpensive to manufacture.

A further object is a printer having a floating decimal point capability.

SUMMARY OF THE INVENTION In brief, my printer is a line printer comprising a set of type wheels that rotate in unison on a common shaft. However, instead of rotating the wheels continuously, I subject them to intermittent motion so that about three-fourths the time they are in movement from one position to the next and during the other one-fourth they are at rest in the successive positions. The printing hammers are actuated when the type wheels are at rest and this provides an important advantage of the invention.

Specifically, since the wheels are at rest in each position for an appreciable length of time, e.g. approximately 10 milliseconds, the timing of the hammers is not critical; nor must the contact with the paper be limited to the extremely short interval required with conventional line printers. This alone permits a much simpler and less costly hammer design and it pretty much eliminates hammer synchronization problems. For example, the printer operations may be timed by a 30 Hz square wave derived from a Hz line voltage with all comparisons, printing, and other preparations occuring during one half cycle. The drum is advanced to the next character position thereafter. Paper is fed simultaneously with the drum advance when required.

The invention also includes a number of other features described below, such as a novel mechanism for advancing the type wheels and feeding paper through the printer. There are also a simple, effective circuit for controlling operation of the printer and a novel floating decimal point arrangement.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a simplified perspective view, partly schematic, of a line printer embodying the invention;

FIG. 2 is a diagram of the power supply and control sections of the electric circuitry employed in the printer;

FIG. 3 is a circuit diagram of the position sensor, comparator and related circuit elements;

FIG. 4 is a timing chart showing the timing of various signals;

FIGS. 5A and 5B are side elevations of the drum indexing mechanism, show the mechanism in its rest and cocked positions;

FIG. 6A is a fragmentary top view of the printer, showing details of the paper advance mechanism;

FIG. 6B is a section along line 6B6B of FIG. 6A;

FIG. 6C is a section along line 6C-6C of FIG. 6B;

FIG. 7A is a side elevation of a printing wheel used in the printer; and

FIG. 7B is a front elevation of the printing wheel.

ILLUSTRATIVE EMBODIMENT OF THE INVENTION As shown in FIG. 1 a line printer embodying the invention includes a drum 10 carrying on its surface type that prints characters on a paper tape 12 in response to the striking action of hammers 14. The drum comprises a set of type wheels 16 mounted on a shaft 18, with the angular positions of the wheels 16 being fixed with respect to the shaft by means of keys 19 that fit a slot 20 in the shaft. The characters to be printed are selected by an input device 22 which provides character-selecting signals to a set of comparators 24.

The drum 10 is rotated in step-wise fashion by an indexing mechanism generally indicated at 26, with the position of the drum after each step being sensed by a position sensor 28 comprising a commutator 30 that rotates with the shaft 20 and applies an electrical signal to successive contacts 32. Each contact 32 corresponds to one of the angular positions of the drum 10, so that a signal from any of the segments 32 indicates that a particular row of characters on the drum is in position for printing onto the paper tape 12. Thus in the drum position shown in FIG. 1, the commutator 30 is in contact with the segment 32;, which provides a signal indicating that the numerals 2 on the respective type wheels 16 are in position to be struck by the hammers 14 through the tape 12 to print 2s on the tape 12.

The position sensor 28 also includes an encoder 34 that converts the signals from the contacts 32 into coded representations of the position of the drum 10 corresponding to the code used in the input device 22.

Thus each time the drum 10 is advanced to a new position, the comparators 24 compare the output of the encoder. 34 with their inputs from the input device 22 and whenever there is matched between the two sets of signals, a comparator 24 energizes a hammer solenoid 36 to impel a hammer 14 against the tape 12 and thereby print a selected character on the corresponding type wheel 16. For example, if the input device 22 has selected a 2 in the first position of a number to be printed it will apply signals corresponding to 2 to the comparator 24 When the position-sensor contact 32 is contacted by the comutator 30, the encoder 34 will transmit a corresponding signal to all of the comparators 24 and since this signal will match the input to the comparator 24,, the latter comparator will energize the solenoid 36 to drive the hammer 14, toward the type wheel 16, and thereby print the numeral 2 on the tape 12 as indicated in FIG. 1.

Similarly if the signal received by the comparator 24 from the input device 22 corresponds to the numeral 5 for example, the comparator 24., will energize the corresponding solenoid 32 when the comutator 30 reaches the segment 32,, i.e., when the 5s are in the printing position, thereby printing the number 5" in the fourth digit position on the tape 12.

After the drum 10 has made a complete revolution, an entire line of characters will have been printed in this manner. A tape advance unit 38 steps the tape 12 in the direction of the arrow 40 to then position the tape for the printing of the next line of characters. The advance unit 38 includes an indexing mechanism, fragmentarily indicated at 42, having a rachet and pawl arrangement similar to that of the drum indexing mechanism 26 which I shall now describe in detail.

As shown in FIGS. 5A and 5B, the indexing mechanism 26 includes a pair of rachet wheels 44 and 46 keyed to the shaft 18 (FIG. I) and a pair of pawls 48 and 50 that interact with the rachet wheel 44. The pawl 48, which is a driving pawl, is pivoted at the end of a pivot link 52, the link 52 in turn being mounted on a pivot 54. The plunger 56 of a solenoid 58 is pivotedly connected to the link 52 a short distance above the pivot 54.

FIG. 5A depicts the indexing mechanism 26 at rest, with the pawl i engaging the back of a rachet tooth 44a and the front of a tooth 44b. When the shaft 18 and drum 10 mounted thereon are to be indexed, the solenoid 58 is momentarily energized to retract the plunger 56 and with it the link 52, as shown in FIG. 4B. This withdraws the pawl from the front surface of the tooth 44b so that it drops onto the front surface of the next tooth 44c, under the action of leaf spring 60. During, retraction of the pawl 48, backward movement of the rachet wheel 44 is prevented by a pawl 50 which engages the back surface of a tooth 44d. Then, when the solenoid 58 is de-energized, a spring 62, connected to the link 52 below the pivot 54, moves the link and pawl 48 forward against the back surface of the tooth 44b, thereby driving the tooth 44b forward and indexing the rachet wheel 44 by one notch. The tooth 44b then occupies the position of the tooth 44a in FIG. 5A and the drum then rests in its next angular position. The keying engagement between the rachet wheel 44 and shaft 18 ensures that each time the drum It) comes to rest, a row of type characters on the drum will be in the printing position (FIG. 1).

In addition to this typical rachet operation, the indexing mechanism 26 includes an over run prevention arrangement involving a detent 64 on the link 52. The detent 64 is positioned so as to engage the radial front surface of a tooth on the rachet wheel 46, as shown in FIG. 5A, whenever the indexing mechanism 26 is in its rest position. As shown in FIG. 5B, the detent 64 is clear of the rachet wheel 46 as the, pawl 48 begins its forward indexing movement. However, near the end of the forward stroke, the detent 64 enters a position above the radial surface of one of the wheel 46 teeth and when the pawl reaches the end of the forward stroke, as shown in FIG. 5A, the radial tooth surface abuts the detent. The detent 64 thus prevents overrunning of the wheels 44 and 46 and drum 10 beyond the rest position.

At the same time the pawl 50, which is positioned against the wheel 44 by a spring 66, prevents the wheel 46 from bouncing off the detent 64 with a corresponding backward movement of the drum 10. The drum is thus securely and accurately locked in position each time it comes to rest.

The importance of overrun prevention will be readily appreciated when one considers the relatively high indexing rate of the mechanism 26. The drum 10 is stepped 30 times per second, so that it has a fairly high angular velocity during the short portion of each indexing cycle that the stepping motion actually takes place. Without overrun prevention the inertia of the drum may cause it to move beyond the rest position of the pawl 48, thereby complicating the task of having the drum stop at a pre-determined position for printing.

FIGS. 6A, 6B and 6C show the details of the tape advance unit 38. As shown in FIG. 6A the advance unit 38, the drum and a tape supply spool 69 are supported between a pair of side plates 66 and 67 attached to a front panel 68. With reference to FIGS. 6A and 6B, the tape 12 passes from the spool 69 forwardly over a bar 70, down around a pair of drive rolls 71 and again forwardly between a bottom plate 72 and a guide plate 73. The tape continues beneath the drum 10 and then out through an aperture 68a in the front panel 68.

The guide rolls 71 extend downwardly through apertures 73a in the plate 73 (see FIG. 6C, for example), with leaf springs 74 attached to the bottom plate 72 exerting an upward force against rolls 71 through apertures 72a in the bottom plate. The drive rolls are fixed to a shaft 75 that is stepped by the indexing mechanism 42 (FIG. 1) to move the tape 12 forward each time a line has been printed thereon. The indexing mechanism has the same construction as the mechanism 26 illustrated in FIGS. 5A and 5B.

Ordinarily one might expect to pass the tape 12 between the nip of a pair of drive rolls to ensure positive, accurate movement of the tape 12. However the combination of the upward force exerted on the-tape 12 by the springs 74, the extensive peripheral contact between the drive rolls and the tape 12 provided by the position of the bar 70, and a slight drag on the spool 69 combine to provide the requisite tape drive function with a less complicated and less expensive arrangement. Moreover the tape is easily threaded through the advance unit 38 and the rest of the printer, a feature that should be appreciated in the context of the small size of the printer and the relatively inaccessible portions of the printer through which the tape passes.

Threading of the tape is facilitated by the removability of the bar 70 which rests in slots 66a and 67a in the side plates 66 and 67. With the bar 70 removed the tape can easily be passed forward beneath the rolls 71 and on through the printer, after which a bight is formed in the tape to the rear of the drive rolls, the bar is dropped back into the slots 66a and 67a beneath the bight and finally the spool 69 is reversely turned to take up the slack.

As shown in FIG. 2 the printer includes a power supply 86 and a control section 88. The power supply 86 includes the usual connection 90 to the mains, an input transformer 92, a rectifier 94 and a voltage regulator 96 that derives from the output of the rectifier a regulated supply for the various logic circuits used in the printer. A power delay circuit includes a delay unit 100 that emits a signal a predetermined interval after the power is turned on, this signal turning on a switch 102 connecting a solenoid return line 104 to a common conductor 106. The output of the delay unit 100 is differentiated to provide a pulse when the switch 102 is turned on, this pulse serving as an automatic drum homing command whose function will now be described.

The drum 10 (FIG. 1) should begin its rotation from a home position for each line that is printed, thereby insuring that the drum makes a complete revolution to bring each character on its periphery into position for possible printing on the paper tape 12. When the printer is first turned on, however, the drum may, for one reason or another, be in some other position. The control section 88 responds to the drum homing command by bringing the drum to the home position.

More specifically, the homing command is applied to the set input terminal of a flip-flop 108 by way of an OR circuit 1 12 and directly to the set terminal of a hammer inhibit flip-flop 110. The resulting ONE output of the flip flop 108 activates a system clock 116. The clock 1 16 generates a square wave clock signal in response to the output of a pulse generator 118 which is triggered by the 60 Hz a.c. The frequency of the clock signal is one-half the a.c. frequency, i.e., 30 Hz in this example of the invention. The generator 118 includes a phasing network that causes each pulse to be centered on the positive peaks of alternate A.C. signals and corresponding pulses from the pulse generator 118, as seen in the curves 117 and 119 ofFIG. 4.

The clock signal is applied to a silicon controlled rectifier 120 in series between the drum advance solenoid 58 (FIGS. 1 and 2) and the solenoid return line 104. Thus during each positive half cycle of the clock signal the silicon controlled rectifier 120 is turned on so as to complete a circuit from a power line 121, through the solenoid 58, the return line 104 and switch 102 back to the common conductor 106. Thus during the middle portion of each positive clock signal half cycle, when the power supply voltage exceeds the breakdown voltage of the silicon controlled rectifier 120, the rectifier conducts, thereby energizing the solenoid 58. As soon as the supply voltage drops below the breakdown level the rectifier 120 stops conducting, thereby deenergizing the solenoid 58. The drum indexing mechanism 26 (FIG. 1) thereupon operates in the manner described above to index the drum 10 one step.

Successive clock cycles or pulses cause the drum to continue to step in response to current pulses through the solenoid 5 8. The timing of these pulses is shown in the curve 122 of FIG. 4. Finally, with reference to FIG. 3, the commutator 30, contacts a segment 32,, thereby grounding that segment and emitting a stop signal through an inverter 124. The segment 32 is not one on which the commutator stops. Rather it is contacted by the commutator 30 as the latter moves from the drum rest positions corresponding to the segments 32 and 32 Thus the stop signal occurs between two pulses from the system clock as will be seen from a comparison ofthe curves 125 and 119 in FIG. 4.

Returning to FIG. 2, the stop signal is applied to the reset input terminal of the flip-flop 108. The ONE output of the flip-flop 108 thus ceases, thereby inhibiting the clock 116 and terminating the system clock signal. Actuation of the solenoid 58 thus ceases and the commutator 30 comes to rest on the segment 32 (FIGS. 1 and 3) to which the last action of the drum advance mechanism has brought it. This is the home position.

When the input device 22 has an output to be printed by the printer, it provides the corresponding character signals to the comparators 24 (FIGS. 1 and 3) and it then transmits a print command to an AND circuit 126. Assuming that a print enable switch 128 has been closed, the AND circuit 126 transmits its signal through the OR circuit 1 12 to set the flip-flop 108. This turns on the system clock 116 to energize the drum advance solenoid 58 and index the drum 10 as described above. Also, since the inhibit flip-flop 110 has not been set, its ZERO output enables an AND circuit 130 to pass the clock signal to the comparator section for use as a hammer clock signal.

Thus, with reference to FIG. 3, as the commutator 30 indexes to the segment 32, and succeeding segments, the encoder 34 applies a succession of segment-identifying signals to the comparators 24. Each time one of these signals matches the input to the comparator from the input device 22, the comparator emits a comparison signal to a corresponding AND circuit 132. This permits the AND circuit 132 to pass the hammer clock signal to a silicon controlled rectifier 134 in series with the corresponding hammer solenoid 36. The rectifier 134 thus conducts, thereby passing a current pulse through the solenoid 36 from the power supply to the solenoid return line 104. The energized solenoid. thereupon drives a printing hammer 14 (FIG. 1) to print the selected character.

For example, suppose that the numeral 7 is to be printed in the second digit position, i.e., by the printing wheel 16, of FIG. 1. When the commutator 30 (FIGIB) reaches the segment 32-,, the comparator 24 will emit a signal indicating a match between its inputs from the input device 22 and the encoder 34. This signal will enable the AND circuit 132 to pass the positive half cycle of the hammer clock signal to the silicon controlled rectifier 134 thereby passing a current pulse through the solenoid 36 This will cause the hammer 14 (FIG. 1) to strike the paper tape 12 against the printing wheel 16;, thereby printing the numeral '7 on the tape 12.

The pulse through the solenoid 36 as shown in the curve 136 of FIG. 4, and the drum advance solenoid pulse (curve 122) occur simultaneously when the drum is at rest. Hence, the drum advance solenoid 58 (FIG. 1) has been energized preparatory to advancing the drum to its next position. .When SCR 120 stops conducting, the solenoid 58 is deenergized; and the spring 62 (FIG. 5B) advances the drum one character positron.

With further reference to FIG. 3, when the commutator contacts the next segment 32 the latter segment emits a paper advance signal to the control section 88 by way of an inverter 138. As shown in FIG. 2, this enables the AND circuit 131 to pass the clock signal to a silicon controlled rectifier 142 connected in series with a solenoid 144 in the paper advance unit 38 (FIG. 1) simultaneously with solenoid 58. After the resulting current pulse through the solenoid 144, the paper tape 12 advances into position for printing of the next line while the drum 16 is advanced simultaneously to the home position.

As indicated by the curve 145 of FIG. 4, the pulse applied to the solenoid 144 occurs around the middle of a positive half cycle of the clock signal. During the same interval the drum advance solenoid 58 is energized as indicated in curve 122 so that shortly thereafter the drum indexing mechanism 26 (FIG. 1) advances the drum to the home position. As described above with reference to FIG. 3, this moves the commutator 30 over the stop segment 32,, thereby emitting a stop.

signal which resets the flip-flop 108 and turns off the system clock 116. The printer is now ready to receive the information and print command for the next line to be printed on the tape 12.

More particularly, with further reference to FIG. 2, when the flip-flop 108 is set, either by a print command or a drum homing command, the corresponding ONE output of the flip-flop 108 is applied to a buffer 136 that emits a corresponding hold signal to the input device 22. This indicates to the device 22 that it should transmit no further signals to the printer and should not change any signals that have been applied to the comparators 24 (FIG. ll) When the stop signal is emitted by the position sensor 28 after the printing portion of the drum cycle has been completed, the resulting resetting of the flip-flop 108 terminates the hold signal. In response, the input device 22 can now transmit a new set of signals to the printer.

It will be desirable on occasion to advance the tape 12 several lines without imprinting any characters. For example, after a number of lines have been printed one may desire to retrieve the printed information from the printer and the paper must .then be indexed a number of lines to move the last printed line beyond the front panel 68 (FIG. 68) before the tape is severed. For this purpose I prefer to include a paper advance switch 148 whose output is applied to an AND circuit 150. The other input for the AND circuit 150 is the output of a flip-flop 152 that receives the pulses from the generator 118 at a toggle input terminal. The flip-flop 152 thus generates a signal similar to the system clock signal and with the switch 148 actuated, the AND circuit 150 applies this signal to the silicon controlled rectifier 142. As a result, the solenoid 144 is repeatedly energized at the clock rate to rapidly advance the paper tape until the switch 1 is released.

Similarly, when the input device is an instrument, e.g. a voltmeter, whose output is not to be continually automatically printed, the print command signal may be applied manually from time to time by means of a print command switch 154. Actuation of the switch 154 results in the printing sequence described above.

Preferably, the printer is also provided with a floating decimal point simply by including the decimal point as one of the characters on some or all of the printing wheels 16 (FIG. 1). Additionally, I have arranged the system so that the wheel that prints the decimal point can also print another character, as contrasted with the usual arrangement in which the decimal point requires a full character position. This requires complex control circuitry to shift the data to those columns to the left or right of the column randomly selected for decimal point printing.

To understand how this is accomplished, refer first to FIGS. 7A and 7B. As shown therein a printing wheel 16 comprises a rim 156 connected to a hub 158 by a radially-extending web 160. The wheel is integrally molded, with the hub 160 having an aperture 162 for the shaft 18 (FIG. 1 The outer surface of the rim 156 has a series of facets 166, each of which corresponds to one of the positions of the printing drum. The type for one of the characters is molded onto each of the facets.

As shown in FIG. 7B, the facet 166 following the facet 166, carrying the type for the character 0, carries the type for the decimal point as indicated at 168. The decimal point 168 is positioned to the left of the remaining characters all of which are slightly offcenter to the right. Thus, if one of the other characters, e.g. a 7" is printed by the illustrated wheel 16, the wheel can subsequently print the decimal point without striking over the 7.

The circuitry for decimal point printing is illustrated in FIG. 3. A decimal point signal from the input device 22 is applied to one of a set of AND circuits 168 corresponding to the respective printing wheels. When the commutator 30 reaches the segment 32 during each rotation of the drum 10, the output of the inverter 124 also serves as a decimal enable signal that is applied to each of the AND circuits 168. The particular AND circuit that has received the decimal signal from the input device 22 will thereupon emit a corresponding signal to the silicon controlled rectifier 134 associated with the selected printing wheel, thereby causing the associated printing hammer to strike and print the decimal in the manner described above.

In this connection it will be noted that the selected printing wheel 16 (FIG. 1) may already have printed another character, e.g. the numeral 7 in the example above, before the decimal point is imprinted. The corresponding timing is shown in the curve 136 of FIG. 4, which indicates the printing of a 7 when the drum is in its 7 position and printing of the decimal point when the drum is in the eleventh position.

While the printer specifically described above is arranged to print a set of numerals on a narrow paper tape, it will be apparent that the printing drum can accommodate any desired set of symbols instead of, or in addition to, the numerals. Specifically, the drum can be enlarged to accommodate the entire alphabet as well as the numerals to provide a full alphanumeric printing capability. In addition, the drum can be expanded to print a longer line. Even so, the printer will retain its basic simplicity. This simplicity of design, revolving around the rapid stop-and-go motion of the printing drum facilitates synchronization of hammer action by means of an extremely reliable electrical and mechanical arrangement. The same simplicity of design results in low cost relative to the capability of the printer.

Numerous modifications can be made with departing the from scope of the invention, although I prefer the arrangement described above. For example, in the indexing mechanism 26 (FIGS. A and 58) one might arrange the solenoid 58 to provide the forward stroke, with the spring 62 retracting the pawl 48. However, this would result in a substantial increase in the duty cycle of (average value) of the solenoid current, with the disadvantages that that would entail.

I claim:

I. In a line printer of the type including i. a printing drum carrying a plurality of type sets having circumferentially extending characters thereon,

ii. means for moving a record medium past said drum,

iii. means for rotating the drum to bring successive characters in each type set into a printing position,

iv. a hammer associated with each type set, each hammer being positioned to imprint on the record medium characters in the associated type set located in printing position,

v. means for comparing the position of the drum with signals from an input device indicating the characters to be printed by the respective type sets, and

vi. hammer actuating means responsive to the outputs of said comparing means to cause said hammers to imprint characters on said medium, the improvement A. in which the drum rotating means comprises:

1. a first ratchet wheel connected to rotate the drum,

2. a second ratchet wheel, connected to prevent rotation of said drum,

3. a pawl connected to engage the first ratchet wheel and rotate the drum through a single type element at each engagement each time the pawl is actuated,

4. a detent movable with said pawl and connected to engage the second ratchet wheel to prevent rotation of said drum after said first wheel has rotated said drum through said type element, and

5. means for intermittently actuating the pawl whereby the drum is rotated when the pawl is activated and is maintained at rest in a stationary position thereafter, said actuating means actuating the pawl only at intervals sufficiently spaced apart to allow said drum to remain in said stationary position for a time interval sufficient to imprint a selected element on said medi- B. including hammer timing means causing said hammers to imprint during said time interval;

C. said record medium moving means advancing said medium only after said drum has been advanced through all the type elements in a set.

2. A line printer, according to claim 1, in which the means for intermittently actuating the pawl includes:

A. a solenoid connected to retract the pawl, and

B. a spring connected to move the pawl forward to drive the first ratchet and thereby rotate the drum.

3. The printer defined in claim 1 in which said timing means includes A. means generating a cyclical clock signal, and

B. means for causing said solenoid to be energized and said hammers to imprint during the same portions of said clock signal.

4. A printer as defined in claim.1, including a commutator associated with said drum for providing an output indicative of the angular position of said drum with respect to a reference position.

5. A printer as defined in claim 4 in which the commutator has a contact for each character in a type set and a separate contact for generating a stop signal to bring said drum to rest at a fixed reference position after a complete rotation thereof.

6. A printer as defined in claim 5 in which one of the commutator contacts corresponds to a decimal point.

7. A printer as defined in claim 1 including:

A. a main power switch actuable to connect the printer to a source of alternating current; and

B. means responsive to the main power switch to cause the indexing means to index said drum to an initial reference position after said switch is actuated.

8. A printer as defined in claim 7 in which the means responsive to the main power switch includes:

A. a bistable device which is set to a first state when the main power switch is actuated; and

B. means responsive to the setting of the bistable device to apply pulse signals to the indexing means to thereby index the drum to said initial position.

9. A. printer as defined in claim 1 in which the hammer actuating means:

A. is energized only when said solenoid is energized,

and

B. is deenergized when said solenoid is deenergized,

C. whereby said hammers imprint characters on the record medium only when said drum is stationary.

10. A printer as defined in claim 1 including:

A. means for indexing the drum to a reference position when the drum has rotated through a line of characters, and

B. means for advancing the record medium while indexing the drum to said reference position.

11. A printer as defined in claim 1 including switch means for energizing the record medium moving means when the switch is actuated, to thereby move said record medium past said drum without imprinting said medium.

12. A printer as defined in claim 1 in which the record moving means, the drum rotating means, and the hammer actuating means are actuated in synchronism with a clock whose rate is synchronized with the line frequency rate of an alternating current power line supplying the printer.

13. A printer as defined in claim 12 in which the clock rate is an integral multiple or submultiple of the line frequency.

14. A printer as defined in claim 12 in which the record moving means, the drum rotating means, and hammer actuating means, are actuated from the source of alternating current only at the zero-crossings of the current, whereby radiated power is minimized.

15. A printer according to claim 1 which the characters in a type set are disposed on individual faces of a polygonal wheel adapated to rotate with a rotatable shaft and connectable thereto by means of a splined fitting, said wheel being thereby oriented in fixed relation to said shaft when so connected.

16. In a line printer of the type including i. a printing drum carrying a plurality of sets of type having circumferentially extending type characters thereon,

ii. means for moving a record medium past said drum,

iii. means for rotating the drum to bring successive characters in each type set into a printing position,

iv. a hammer associated with each type set, each hammer being positioned to imprint on the record medium characters in the associated type set located in printing position,

v. means for comparing the position of the drum with signals from an input device indicating the characters to be printed by the respective type sets, and

vi. hammer actuating means responsive to the outputs of said comparing means to cause said hammers to imprint characters on said medium, the improvement A. including means for intermittently advancing the drum at least one type element at a time and maintaining the drum, after each advance, in a stationary printing position for a time interval sufficient to imprint a selected element on said medium;

B. including hammer timing means causing said hammers to imprint during said time interval;

C. said record medium moving means advancing said medium only after said drum has been advanced through all the type elements in a set;

D. in which at least one of said type sets includes a decimal point at a circumferential position on said drum different from the circumferential position of the other characters in said type set and axially offset from said other characters; and

E. in which said comparing means includes 1. means for sensing the passage of decimal points into said printing position, and

coincidence means associated with each type set and responsive to the coincidence of the output of said sensing means with a decimal point signal and another character signal from an input device for that type set during the printing of a single line of printed characters to cause said hammer to imprint both said decimal point and the other character in a single character position in said line.

17. In a line printer of the type including a rotating type drum with sets of type characters which extend circumferentially thereon, and which are intermittently pressed against a recording medium by a hammer, the improvement A. in which at least one of said type sets includes a decimal point at a circumferential position on said drum different from the circumferential position of the other characters in said type set and axially offset from said other characters,

B. which includes comparing means having 1. means for sensing the passage of said type characters, including said decimal point, into a printing position, and

. coincidence means associated with each type set and responsive to the coincidences of the output of said sensing means with a decimal point signal and another character signal from an input device for that type set during the printing of a single line of printed characters to cause said hammer to imprint both said decimal point and the other character in a single character position in said line.

18. In a line printer of the type including i. a printing drum carrying a plurality of sets of type having circumferentially extending type characters thereon,

ii. means for moving a record medium past said drum,

iii. means for rotating the drum to bring successive characters in each type set into a printing position,

iv. a hammer associated with each type set, each hammer being positioned to imprint on the record medium characters in the associated type set located in printing position,

v. means for comparing the position of the drum with signals from an input device indicating the characters to be printed by the respective type sets, and

vi. hammer actuating means responsive to the outputs of said comparing means to cause said hammers to imprint characters on said medium, the improvement A. including means for intermittently advancing the drum at least one type leement at a time and maintaining the drum, after each advance, in a stationary printing position for a time interval sufficient to imprint a selected element on said medium;

B. including hammer timing means causing said hammers to imprint during said time interval;

C. in which said record medium moving means adferent from the circumferential position of the vancing said medium only after said drum had other characters in the type set. been advanced through all the type elements in a y from Said other characters, and set; 3. selectable for printing in conjunction with D. in which at least one of said type sets includes a another Charade; in the Same y set during character; the Ofa Smgle line. 1. at a circumferential position on said drum dif- 

1. In a line printer of the type including i. a printing drum carrying a plurality of type sets having circumferentially extending characters thereon, ii. means for moving a record medium past said drum, iii. means for rotating the drum to bring successive characters in each type set into a printing position, iv. a hammer associated with each type set, each hammer being positioned to imprint on the record medium characters in the associated type set located in printing position, v. means for comparing the position of the drum with signals from an input device indicating the characters to be printed by the respective type sets, and vi. hammer actuating means responsive to the outputs of said comparing means to cause said hammers to imprint characters on said medium, the improvement A. in which the drum rotating means comprises:
 1. a first ratchet wheel connected to rotate the drum,
 2. a second ratchet wheel, connected to prevent rotation of said drum,
 3. a pawl connected to engage the first ratchet wheel and rotate the drum through a single type element at each engagement each time the pawl is actuated,
 4. a detent movable with said pawl and connected to engage the second ratchet wheel to prevent rotation of said drum after said first wheel has rotated said drum through said type element, and
 5. means for intermittently actuating the pawl wherebY the drum is rotated when the pawl is activated and is maintained at rest in a stationary position thereafter, said actuating means actuating the pawl only at intervals sufficiently spaced apart to allow said drum to remain in said stationary position for a time interval sufficient to imprint a selected element on said medium, B. including hammer timing means causing said hammers to imprint during said time interval; C. said record medium moving means advancing said medium only after said drum has been advanced through all the type elements in a set.
 2. A line printer, according to claim 1, in which the means for intermittently actuating the pawl includes: A. a solenoid connected to retract the pawl, and B. a spring connected to move the pawl forward to drive the first ratchet and thereby rotate the drum.
 2. a second ratchet wheel, connected to prevent rotation of said drum,
 2. axially offset from said other characters, and
 2. coincidence means associated with each type set and responsive to the coincidences of the output of said sensing means with a decimal point signal and another character signal from an input device for that type set during the printing of a single line of printed characters to cause said hammer to imprint both said decimal point and the other character in a single character position in said line.
 2. coincidence means associated with each type set and responsive to the coincidence of the output of said sensing means with a decimal point signal and another character signal from an input device for that type set during the printing of a single line of printed characters to cause said hammer to imprint both said decimal point and the other character in a single character position in said line.
 3. selectable for printing in conjunction with another character in the same type set during the printing of a single line.
 3. a pawl connected to engage the first ratchet wheel and rotate the drum through a single type element at each engagement each time the pawl is actuated,
 3. The printer defined in claim 1 in which said timing means includes A. means generating a cyclical clock signal, and B. means for causing said solenoid to be energized and said hammers to imprint during the same portions of said clock signal.
 4. a detent movable with said pawl and connected to engage the second ratchet wheel to prevent rotation of said drum after said first wheel has rotated said drum through said type element, and
 4. A printer as defined in claim 1, including a commutator associated with said drum for providing an output indicative of the angular position of said drum with respect to a reference position.
 5. A printer as defined in claim 4 in which the commutator has a contact for each character in a type set and a separate contact for generating a stop signal to bring said drum to rest at a fixed reference position after a complete rotation thereof.
 5. means for intermittently actuating the pawl wherebY the drum is rotated when the pawl is activated and is maintained at rest in a stationary position thereafter, said actuating means actuating the pawl only at intervals sufficiently spaced apart to allow said drum to remain in said stationary position for a time interval sufficient to imprint a selected element on said medium, B. including hammer timing means causing said hammers to imprint during said time interval; C. said record medium moving means advancing said medium only after said drum has been advanced through all the type elements in a set.
 6. A printer as defined in claim 5 in which one of the commutator contacts corresponds to a decimal point.
 7. A printer as defined in claim 1 including: A. a main power switch actuable to connect the printer to a source of alternating current; and B. means responsive to the main power switch to cause the indexing means to index said drum to an initial reference position after said switch is actuated.
 8. A printer as defined in claim 7 in which the means responsive to the main power switch includes: A. a bistable device which is set to a first state when the main power switch is actuated; and B. means responsive to the setting of the bistable device to apply pulse signals to the indexing means to thereby index the drum to said initial position.
 9. A printer as defined in claim 1 in which the hammer actuating means: A. is energized only when said solenoid is energized, and B. is deenergized when said solenoid is deenergized, C. whereby said hammers imprint characters on the record medium only when said drum is stationary.
 10. A printer as defined in claim 1 including: A. means for indexing the drum to a reference position when the drum has rotated through a line of characters, and B. means for advancing the record medium while indexing the drum to said reference position.
 11. A printer as defined in claim 1 including switch means for energizing the record medium moving means when the switch is actuated, to thereby move said record medium past said drum without imprinting said medium.
 12. A printer as defined in claim 1 in which the record moving means, the drum rotating means, and the hammer actuating means are actuated in synchronism with a clock whose rate is synchronized with the line frequency rate of an alternating current power line supplying the printer.
 13. A printer as defined in claim 12 in which the clock rate is an integral multiple or submultiple of the line frequency.
 14. A printer as defined in claim 12 in which the record moving means, the drum rotating means, and hammer actuating means, are actuated from the source of alternating current only at the zero-crossings of the current, whereby radiated power is minimized.
 15. A printer according to claim 1 which the characters in a type set are disposed on individual faces of a polygonal wheel adapated to rotate with a rotatable shaft and connectable thereto by means of a splined fitting, said wheel being thereby oriented in fixed relation to said shaft when so connected.
 16. In a line printer of the type including i. a printing drum carrying a plurality of sets of type having circumferentially extending type characters thereon, ii. means for moving a record medium past said drum, iii. means for rotating the drum to bring successive characters in each type set into a printing position, iv. a hammer associated with each type set, each hammer being positioned to imprint on the record medium characters in the associated type set located in printing position, v. means for comparing the position of the drum with signals from an input device indicating the characters to be printed by the respective type sets, and vi. hammer actuating means responsive to the outputs of said comparing means to cause said hammers to imprint characters on said medium, the improvement A. including means for intermittently advancing the drum at least one type element at a time and maintaining the drum, after each advance, in a stationary printing position for a time interval sufficient to imprint a selected element on said medium; B. including hammer timing means causing said hammers to imprint during said time interval; C. said record medium moving means advancing said medium only after said drum has been advanced through all the type elements in a set; D. in which at least one of said type sets includes a decimal point at a circumferential position on said drum different from the circumferential position of the other characters in said type set and axially offset from said other characters; and E. in which said comparing means includes
 17. In a line printer of the type including a rotating type drum with sets of type characters which extend circumferentially thereon, and which are intermittently pressed against a recording medium by a hammer, the improvement A. in which at least one of said type sets includes a decimal point at a circumferential position on said drum different from the circumferential position of the other characters in said type set and axially offset from said other characters, B. which includes comparing means having
 18. In a line printer of the type including i. a printing drum carrying a plurality of sets of type having circumferentially extending type characters thereon, ii. means for moving a record medium past said drum, iii. means for rotating the drum to bring successive characters in each type set into a printing position, iv. a hammer associated with each type set, each hammer being positioned to imprint on the record medium characters in the associated type set located in printing position, v. means for comparing the position of the drum with signals from an input device indicating the characters to be printed by the respective type sets, and vi. hammer actuating means responsive to the outputs of said comparing means to cause said hammers to imprint characters on said medium, the improvement A. including means for intermittently advancing the drum at least one type leement at a time and maintaining the drum, after each advance, in a stationary printing position for a time interval sufficient to imprint a selected element on said medium; B. including hammer timing means causing said hammers to imprint during said time interval; C. in which said record medium moving means advancing said medium only after said drum had been advanced through all the type elements in a set; D. in which at least one of said type sets includes a character; 